Break wallets encrypted with a passphrase, or crack private keys? The former is possible for weak passphrases, but if the latter is ever possible that's a Very Bad Thing for Bitcoin.
It may happen eventually, but hopefully we will have migrated to stronger crypto by then. I do wonder what will happen to abandoned coins at that point. Will they become invalid after a migration period, or will they be up for grabs to the fastest cracker?
I meant to crack private keys, it will eventually become viable to spend resources researching/developing an attack vector to break those abandoned wallets. Migration to better cryptos will certainly happen, but remember that the lost wallets won't be upgraded, because they're, well, lost.
Making them invalid would be self-defeating to bitcoin, but it's not bitcoin's fault that some day abandoned wallets will become breakable, there will never be an everlasting bullet-proof encryption, and my point is: It won't happen tomorrow, it may take tens or hundreds of years, but it will certainly happen.
Agreed, though I'm not sure Bitcoin's users/miners wouldn't "vote" to eventually invalidate unmigrated wallets. There's a tradeoff between accidentally destroying someone's legitimate wealth (e.x. a nLockTime'd transaction left as an inheritance) vs allowing crackers to eventually claim potentially enormous abandoned wallets (worth billions, if not trillions of dollars in the future). But remember those unmigrated wallets would also be equally vulnerable to cracking as abandoned wallets.
Perhaps there could be a solution whereby a nLockTime'd transactions could be presented during the migration period. I haven't thought through the details.
Either way, treasure hunting in the future will almost certainly be of the digital variety.
It may happen eventually, but hopefully we will have migrated to stronger crypto by then. I do wonder what will happen to abandoned coins at that point. Will they become invalid after a migration period, or will they be up for grabs to the fastest cracker?